x86/svm: Set IBRS value on VM entry and exit

CVE-2017-5715 (Spectre v2 Intel)

Set/restore the guests IBRS value on VM entry. On VM exit back to the
kernel save the guest IBRS value and then set IBRS to 1.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
(backported from commit ae47b6df435ae255747a9aa1a5520bd9ef01005f)
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

arch/x86/kvm/svm.c

@@ -139,6 +139,8 @@ struct vcpu_svm {
 
 	u64 next_rip;
 
+	u64 spec_ctrl;
+
 	u64 host_user_msrs[NR_HOST_SAVE_USER_MSRS];
 	struct {
 		u16 fs;
@@ -3053,6 +3055,9 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	case MSR_VM_CR:
 		msr_info->data = svm->nested.vm_cr_msr;
 		break;
+	case MSR_IA32_SPEC_CTRL:
+		msr_info->data = svm->spec_ctrl;
+		break;
 	case MSR_IA32_UCODE_REV:
 		msr_info->data = 0x01000065;
 		break;
@@ -3189,6 +3194,9 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 	case MSR_VM_IGNNE:
 		vcpu_unimpl(vcpu, "unimplemented wrmsr: 0x%x data 0x%llx\n", ecx, data);
 		break;
+	case MSR_IA32_SPEC_CTRL:
+		svm->spec_ctrl = data;
+		break;
 	default:
 		return kvm_set_msr_common(vcpu, msr);
 	}
@@ -3827,6 +3835,9 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
 
 	local_irq_enable();
 
+	if (ibrs_inuse && (svm->spec_ctrl != FEATURE_ENABLE_IBRS))
+		wrmsrl(MSR_IA32_SPEC_CTRL, svm->spec_ctrl);
+
 	asm volatile (
 		"push %%" _ASM_BP "; \n\t"
 		"mov %c[rbx](%[svm]), %%" _ASM_BX " \n\t"
@@ -3903,6 +3914,12 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
 	/* Eliminate branch target predictions from guest mode */
 	vmexit_fill_RSB();
 
+	if (ibrs_inuse) {
+		rdmsrl(MSR_IA32_SPEC_CTRL, svm->spec_ctrl);
+		if (svm->spec_ctrl != FEATURE_ENABLE_IBRS)
+			wrmsrl(MSR_IA32_SPEC_CTRL, FEATURE_ENABLE_IBRS);
+	}
+
 #ifdef CONFIG_X86_64
 	wrmsrl(MSR_GS_BASE, svm->host.gs_base);
 #else